1. Field of the Invention
The present invention relates to a method for recording and reproducing data on an optical disk of a phase-change type, particularly to the method for writing, erasing and reading data on the optical disk which is capable of being over-written.
2. Description of the Prior Art
E-DRAW (Erasable--Direct Read After Write) optical disks were recently developed, some of which are subject to practical applications. Such optical disks are classified into two types of a magneto-optical disk and an optical disk of a phase-change type. In the latter, such property of an alloy is utilized that the reflection factor thereof changes due to the phase transition thereof between phases of crystalline and amorphous. A structure of the optical disk of the phase-change type is illustrated in FIG. 1, in which a substrate disk 31 is made of acrylic or polycarbonate resin. On the surface of the substrate disk 31, a pregroove 32 is formed so as to be used for the tracking servo of a light beam. A recording film 33 is deposited on the pregroove 32 through the vacuum evaporation process or the sputtering deposition process. A protective film 34 is deposited on the recording film 33. Another protective film 34 (which is not shown) may be formed between the substrate disk 31 and the recording film 33.
In such a structure of the optical disk of the phase-change type 30, a phase condition of the recording film 33 after its deposition is a kind of an initial amorphous state. However, this initial amorphous state of the recording film is different from amorphous states which will be generated due to the phase-change thereof caused by the irradiation of the light beam thereon. When data are written in the optical disk 30, the initialization of the optical disk is performed in such a manner that an initializing light beam IL is irradiated onto the recording film 33 along with the pregroove 32 as seen from FIG. 2AA. By the irradiation of the initializing light beam IL at a high power level in the form of a spot, a portion of the recording film 33 within the spot in the pregroove 32 is heated above the melting point to take an amorphous state by quenching or rapid cooling. At the same time, the neighboring portions at both sides of the irradiation portion are crystallized because they are heated between the melting point and the crystallization transition temperature and thereafter annealed or slowly cooled down. As a result, the initialized region IR includes a central amorphous region CR on the pregroove 32 and a pair of crystallized neighboring regions NR which sandwich the central amorphous region as seen from FIG. 2AA.
The inventors of the present Application revealed through various experiments that when data are over-written on the initialized region IR by irradiating of a writing light beam WL having an intensity lower than that of the initializing light beam IL, peripheral parts PP of the crystallized neighboring regions which project from the initialized region IR remain as unerased residues on the track of the phase-change type optical disk during over-write recording, as shown in FIG. 2AA. FIG. 2B is a photograph showing such and actual state of the surface of the recording film of the optical disk as described. In other words, the initialized region IR is crystallized by an erasing light beam of a medium power level before incident of the writing light beam of a high power level. The writing light beam WL is irradiated on the initialized region IR in response to data to be over-written on the optical disk, so that the data are over-written on the crystallized region as a row of local amorphous domains of written spots (FIG. 2B). The diameters of spots of these initializing, erasing and writing light beams are usually set to be kept substantially the same each other. However, the neighboring portions of the local amorphous domains are so widely crystallized as to expand beyond from the initialized region IR as shown in FIGS. 2AA and 2B. In this way, the peripheral parts PP of the crystallized neighboring regions remain as the unerased residues. In the over-write recording process, the unerased residues adversely influence the reproducing of the data written on the optical disk so as to generate noise.